Ultrashort Picosecond Ion Pulses via Laser-Stimulated Desorption for Time-Resolved Ion-Solid Interaction Studies

Abstract

We present a laser-stimulated desorption tech- nique to generate tunable picosecond ion pulses, ideally suited for direct investigation of ion-solid interactions in pump-probe experiments. The sub-100 ps ion pulse duration and inherent syn- chronization between ion and optical pulses en- ables real-time probing of surface dynamics, de- fect formation, and energy dissipation pathways following ion impact. The ion source consists of an electrochemically etched tungsten nanotip (→150 nm apex radius) biased at +6.5 kV in an ultrahigh vacuum cham- ber. Introducing a background gas with a typical partial pressure of → 5 ↑ 10→7 mbar allows ad- sorbates to form on the tip surface. A 259 nm ul- traviolet laser pulse with pulse length of 280 fs, focused onto the tip apex with a 3.6μm beam diameter, ionizes the adsorbates. While gas- phase ionization requires high optical power den- sities, our approach achieves ionization with only →15 nJ/pulse, corresponding to → 1011 W/cm2. The resulting ions are extracted and travel trough a 2.6cm flight region before impacting a tar- get with kinetic energy of 8.5keV. A fraction of the same laser pulse is split off and could be passed through an optical delay stage, en- abling adjustable pump-probe delays over sev- eral nanoseconds with picosecond resolution. To determine the ions time-of-flight (TOF) dis- tribution, the split-off laser pulse is picked up by a pin diode, which acts as starting signal, while the target is replaced with a microchannel plate detector, which functions as the stop signal. This setup yields ion TOF spectra, as seen in Figure 1. The shape of an ion pulse follows a lognormal distribution, due to the minimum flight time be- ing constrained by the shortest possible ion tra- jectory. The ion pulse width, defined as ion ar- rival time jitter, could therefore be reduced by geometric filtering of flight paths as described in [1]. (a) D+ (b) N+ Kr+ 84 ps FWHM 50 150 250 100 300 500 700 arbitrary time-of-flight [ps] counts [arb. units] Figure 1: Ion TOF spectra: (a) Hydrogen ion peak with a lognormal fit, illustrating the sub- 100 ps pulse width. (b) Different ionic species, horizontally shifted to align their peaks. The measured TOF spectra are also greatly influ- enced by intrinsic timing jitter from the detectors and used electronics. A typical TOF distribution of hydrogen ions is shown in Figure 1(a), featur- ing a full-width at half-maximum (FWHM) be- low 100 ps. By changing the working gas, a variety of ionic species can be generated, as demonstrated in Fig- ure 1(b), where deuterium and nitrogen are intro- duced into the vacuum chamber. However, the gas must be capable of adsorbing onto the tung- sten tip; thus, noble gas ions cannot be produced under room-temperature conditions. References [1] Mihaila MCC et al. Phys. Rev. Res. 6 3 (2024)